A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers. In steppers each target portion is irradiated by exposing an entire pattern onto the target portion at one time. Another known lithographic apparatus include so-called scanners. In scanners each target portion is irradiated by scanning the pattern through a projection radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this given direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
A lithographic apparatus makes measurements before starting and/or during transfer of the pattern onto the substrate. An example measurement is the position of the substrate relative to a reference frame or a projection system through which the pattern is projected. Measurements can be made using a measurement radiation beam.
The rate at which the lithographic apparatus applies the desired pattern on the substrate, known as throughput, is a major performance criteria in lithography apparatus. A faster throughput is desirable. Throughput is dependent on multiple factors. One factor on which throughput is dependent is the speed at which transfer of the pattern onto the substrate takes place. Another factor on which throughput is dependent is the speed at which measurements needed prior to transfer of the pattern can be taken. Therefore, it is beneficial to have high moving speeds of the substrate during transfer of the pattern onto the substrate and during measurement. However, it is important to maintain accuracy of measurements and transfer of the pattern at the high moving speeds.
Measurement radiation beams in the lithographic apparatus pass through gas, referred to as ambient gas. Local fluctuations in the characteristics of an ambient gas can affect a measurement radiation beam passing through it. Projection radiation beams can be affected in the same way as measurement radiation beams. Therefore, it is an aim of the present invention to provide an apparatus with reduced fluctuations in the characteristics of an ambient gas at a location through which a measurement radiation beam and/or a projection radiation beam passes.